1. Field of the Invention
The present invention relates to a differential signal receiving device and a differential signal transmission system and, more particularly, to a differential signal receiving device and a differential signal transmission system using a plurality of differential signals.
2. Description of the Related Art
Signal transmission devices that transmit a differential signal through a pair of transmission lines have been used. The transmission of a differential signal through a transmission line pair is effective in reducing noise since it can cancel out the noise with substantially the same amplitude and phase occurring equally on the transmission line pair and suppress electromagnetic interference (EMI: a generic term for electromagnetic disturbance and interruption to an external device).
Japanese Unexamined Patent Application Publication No. 2002-204272 describes and illustrates in FIG. 2 a signal transmission system that superposes a different signal on each of first and second differential signals and transmits the first and second differential signals superposed with different signals through two pairs of transmission lines.
Specifically, a positive phase single-ended signal is input to an intermediate node of resistors placed between the transmission line pair for transmitting the first differential signal, and a reverse phase single-ended signal is input to an intermediate node of resistors placed between the transmission line pair for transmitting the second differential signal. This allows the transmission of two differential signals and one single-ended signal with four transmission lines.
To receive the first differential signal, a differential signal receiving circuit detects a voltage at a terminator placed between the transmission line pair for transmitting the first differential signal. To receive the second differential signal, the differential signal receiving circuit detects a voltage at a terminator placed between the transmission line pair for transmitting the second differential signal.
The positive phase single-ended signal is detected at an intermediate node of the terminator placed between the transmission line pair for transmitting the first differential signal. The reverse phase single-ended signal is detected at an intermediate node of the terminator placed between the transmission line pair for transmitting the second differential signal.
The conventional signal transmission system may transmit a third differential signal instead of the positive phase single-ended signal and the reverse phase single-ended signal. This allows the transmission of three differential signals with four transmission lines.
However, when the conventional signal transmission system transmits the third differential signal instead of the positive phase and reverse phase single-ended signals, the third differential signal is undesirably reflected when it reaches the end of the signal line.
This is because the ends of the signal lines transmitting the third differential signal, which are between the terminator placed between the transmission line pair transmitting the first differential signal and the terminator placed between the transmission line pair transmitting the second differential signal, are open (high impedance) and the impedances of the terminators and the signal lines for the third differential signal do not match.
Further, in the case of transmitting the three differential signals with the four signal lines, an input buffer at the receiving side should extract and amplify the third differential signal in addition to receive the first and second differential signals. It is thus preferred that the input buffer effectively extracts the third differential signal and has an efficient circuit configuration.